1. Field of the Invention
The present invention relates to a scan driving circuit. In particular, the present invention relates to a scan driving circuit capable of reducing power consumption and an electroluminescent display using the same.
2. Description of the Related Art
In general, an electroluminescent (EL) display is a flat display device, where voltage may be applied via drive and scan lines to light emitting layers to form images.
A conventional EL display may be an active matrix type display having a plurality of scan lines, e.g., horizontally arranged rows, a plurality of data lines arranged perpendicularly to the scan lines, and a plurality of pixels arranged at intersection points between data lines and scan lines to form a matrix pattern. The scan and data lines may provide scan and data signals, respectively, to the plurality of pixels via at least one driving circuit.
The conventional driving circuit of an EL display may include a shift register unit having a plurality of stages connected in cascade. The plurality of stages may be supplied with an input signal along with high-level and low-level driving voltages and clock signals to sequentially shift an output signal from one stage to another, i.e., an output signal of each stage may become an input signal of the following stage. Accordingly, the input signal may be shifted and provided through an output of each stage to a respective line, such that a plurality of output signals may be transmitted through a plurality of lines, e.g., scan lines or data lines, to the pixel matrix of the EL display.
Each stage of the conventional driving circuit may include a master-slave flip-flop with an inverter. In other words, when a clock signal is at a low level (‘0’), the flip-flop may receive an input signal but maintain a previous output signal. On the other hand, when the clock changes to a high level (‘1’), the flip-flop may maintain its previous input, i.e., an input received when the clock signal was at the low level (‘0’), while outputting a new output signal with respect to the input signal. Further, the flip-flop operation may include electric current flow in the inverter either through an input transistor or through a load transistor to charge or discharge, respectively, an output terminal of the flip-flop.
However, when the clock signal of the flip-flop of the conventional scan driving circuit is at a low level, the inverter incorporated therein may produce a static current flow and, thereby, increase the overall power consumption of the flip-flop. Such power consumption may be increased even further as the number of the inverters receiving low-level clock signals increases. Further, upon charging of the output terminal, i.e., electric current flow through the input transistor of the inverter, a source-gate voltage of the load transistor may be gradually reduced, thereby decreasing the discharge current therethrough. Such decrease in the discharge current may deteriorate the overall discharge efficiency of the driving circuit.
Additionally, an output voltage of each stage of the conventional driving circuit may be determined by a transistor connected between a power supply VDD and a ground, such that a high output voltage level may be set by a voltage value of the transistor and a low voltage level may be set to be greater than that of the ground by a threshold voltage of an input transistor. However, since levels of an input voltage may be different at every stage of the shift register unit, voltage deviations due to the transistor may affect the output voltage at a low level, thereby triggering incorrect operation of the driving circuit. Moreover, low level voltage deviations due to the transistor may cause deviations in the resistance of the input transistor and inverter, thereby triggering voltage deviation of the high level output voltage as well. Such voltage deviations may increase even further in an organic EL display due to the characteristic high voltage deviations associated with the transistors employed therein.
Accordingly, there exists a need for a driving circuit of an EL display having an improved structure capable of providing reduced power consumption and improved circuit operation and discharge efficiency.